1. Field of the Invention
The present invention relates to a resin composition comprising a polyamide resin and a polyester resin, and a shaped article and a packaging container made of the resin composition. More specifically, the present invention relates to a resin composition produced by melt-blending a specific polyamide resin containing a phosphorus compound, a polyester resin containing an antimony compound and at least one compound selected from the group consisting of inorganic acid salts and hydroxides of the group-1 and -2 metals of the periodic table and the transition metals, the resin composition improving the gas barrier properties which is insufficient in the conventional resin compositions comprising only a polyester resin and eliminating the problem of a darkened appearance in the conventional resin compositions. The present invention also relates to a shaped article and a packaging container made of such a resin composition.
2. Description of the Prior Art
A polyester resin represented by a polymer, such as poly(ethylene terephthalate), which is prepared using an aromatic dicarboxylic acid and an aliphatic diol as monomers (hereinafter referred to as “polyester resin”) has been now widely used as packaging materials such as films, sheets and hollow containers because of its excellency in clearness, mechanical properties, melt stability, solvent resistance, flavor retention, recycling ability, etc. In spite of such excellent properties, the application field of packaging containers made of a polyester resin is limited because the polyester resin is not always sufficient in gas barrier properties against oxygen, carbon dioxide, etc. Improvement of the gas barrier properties of polyester resin has been attempted by vapor-depositing aluminum oxide or silicon oxide onto a shaped article or packaging container made of a polyester resin, or by coating or laminating a resin having gas barrier properties higher than that of a polyester resin onto a shaped article or packaging container made of a polyester resin. However, since a complicated production process is required and the recycling ability or mechanical properties are detracted, the polyester resins improved by such methods have been used in limited fields.
As an alternative method for improving the gas barrier properties of polyester resin which is free from the above problems, cited is a melt blending of a thermoplastic resin having high gas barrier properties into a polyester resin. An ethylene-vinyl alcohol copolymer resin can be mentioned as an example for such a high gas barrier resin. However, the ethylene-vinyl alcohol copolymer resin is less compatible with a polyester resin because of its molecular structure to give a white cloudy resin composition when blended with a polyester resin, thereby spoiling the clearness that is one of the beneficial characteristics of a polyester resin. In addition, since the ethylene-vinyl alcohol copolymer resin is relatively high in crystallizability as compared with a polyester resin, the stretchability of polyester resin tends to be lowered, this making the resin blend inapplicable to the production of packaging containers such as oriented films and blown bottles because the production thereof inevitably requires a stretching process. Further, the ethylene-vinyl alcohol copolymer resin is susceptible to a rapid thermal degradation at an optimum temperature for processing a polyester resin. Thus, the blend of the ethylene-vinyl alcohol copolymer resin raises another problem of reducing the processing stability of polyester resin.
Gas barrier resins other than the ethylene-vinyl alcohol copolymer include polyamide resins represented by Nylon 6, Nylon 66, etc. Of the polyamide resins, poly(m-xylylene diadipamide) prepared by the polymerization of a diamine component mainly comprising m-xylylene diamine and a dicarboxylic acid component mainly comprising adipic acid is particularly excellent in the gas barrier properties. In addition to its high gas barrier properties as compared with those of other polyamide resins, poly(m-xylylene diadipamide) does not detract the processing stability of polyester resin because its glass transition temperature, melting point and crystallizability are close to those of a poly(ethylene terephthalate) which is a typical polyester resin currently being widely used. Therefore, poly(m-xylylene diadipamide) is quite suitable for improving the gas barrier properties of polyester resin. A commercially available poly(m-xylylene diadipamide), for example, MX Nylon (product name) of Mitsubishi Gas Chemical Company, Inc., may contain a phosphorus compound in an amount of several hundred ppm in terms of phosphorus atom to prevent the coloring of the resin during the melt processing.
The production of polyester resin is generally carried out in the presence of a metal catalyst containing germanium atom or antimony atom to increase the polymerization rate of monomers. It has been known in the art that a shaped article made of a polyester resin that is produced using an antimony-bearing catalyst is slightly darkened because of the deposition of metallic antimony by the reduction of an antimony compound. Therefore, to avoid the decrease of the commercial value of shaped article, the darkening is prevented by limiting the use amount of the antimony compound. Even in the case of using a limited amount of the antimony compound, if a shaped article is produced by melt-kneading and molding a polyester resin that is blended with poly(m-xylylene diadipamide) to improve the gas barrier properties, the darkening of the shaped article is considerably enhanced as compared with a shaped article from a polyester resin containing no poly(m-xylylene diadipamide), because the deposition of metallic antimony is promoted by a phosphorus compound in poly(m-xylylene diadipamide). Therefore, the commercial value is much reduced when the shaped article is used as packaging materials or packaging containers for foodstuffs, etc., where the appearance is of extreme importance.
Conventionally proposed is a hollow shaped article excellent in gas barrier properties which is made of a resin composition containing a thermoplastic polyester resin mainly constituted by ethylene terephthalate repeating units and a polyamide resin (for example, Japanese Patent Publication No. 4-54702). This document merely proposes to improve the gas barrier properties by incorporating a polyamide resin, and is quite silent as to the problem addressed in the present invention, i.e., the prevention of darkening of shaped articles by the action of phosphorus atom in a polyamide resin. Also proposed is a method of producing a heat-resistant synthetic resin bottle from a poly(ethylene terephthalate) resin blended with a nylon mainly constituted by m-xylylene diamine and adipic acid (for example, Japanese Patent Publication No. 6-78094). This document relates to the improvement of the production method of heat-resistant bottles, and there is nothing about the prevention of darkening of shaped articles by the action of phosphorus atom in a polyamide resin.